Sophisticated elevator systems of the type in which a plurality of cars serve a plurality of floors of a building in response to a group controller generally comprise one of two diverse types. A first type of elevator system is zone-controlled: the building is divided into a plurality of zones, each zone having one or a number of floors in it, there being approximately the same number of zones as there are elevator cars serving the building. The group controller recognizes zones having no cars in them and forces cars into the zones, and hall calls within the zones are normally to be handled by the car located within the zones. Such systems have a drawback in that once a car commences servicing calls within its zone, calls in the opposite direction or calls behind the travel of the car are no longer serviceable by it. Therefore, backup modes and optional features are generally provided to call cars from other zones to assist a zone in which the calls are not capable of being responded to in a desirable fashion, or to fill a preferred zone at the expense of a less preferred zone when there is no car available and unassigned which can be forced to the zone. In addition, once any zone requires additional cars assigned to it to assist in handling the service demand in that zone, backup modes of operation are employed, which cause failure of the system to adequately handle other zones in the building. In such case, even a further degree of backup mode is frequently required so as to unequivocally command a car, which may be needed elsewhere, to answer a hall call which has been outstanding for an unacceptably long time.
Such zone-controlled systems also require additional features or modes of operation in order to ensure, in the event that one of the cars normally available for assignment to a zone is out of service, that among the zones which are serviced, the lobby or other main landing is included.
To overcome certain of the difficulties of zone-controlled systems, other systems employ methods of assigning calls to cars, such as by calculation of the amount of time it is estimated for a car to reach a floor landing at which a call is outstanding, taking into account the service which must be performed by the elevator between its present position and reaching the floor landing for the call under consideration. Heretofore, systems of the prior art have allocated the calls to the cars based upon conditions which exist at the time each hall call is first assigned to a car, the assignment in most cases being unequivocal, unless the call remains unanswered after a predetermined, unacceptable period of time. In such cases, the hall call is frequently treated as a new call and a new assignment is found for it. In other cases, the hall call is reconsidered for assignment to other cars, but the comparison made between reconsidered assignments and the original assignment is based upon the factors which control the original assignment, rather than current factors for the original car.
In both of these types of systems, consideration has previously been given to the alignment of a car with a call, without much consideration being given to other system needs, expected future service, and the like.
The shortcomings of the zone-controlled and the call assignment systems known to the prior art have recently been overcome by inventions disclosed and claimed in commonly owned copending U.S. patent applications filed on even date herewith by Bittar, Ser. No. 099,790 and Ser. No. 099,945. By employing the inventions in both of said copending applications, a system which assigns calls to cars, based upon system response factors in addition to the delay of a given car in reaching the floor landing at which the hall call to be assigned is located, on a cyclic basis which recurs several times per second, continuously assigning each unanswered hall call to any car available for group control in the answering of hall calls, eliminates the difficulties of providing hall call answering service by a plurality of elevators in a building. However, an additional drawback of an elevator system of the type that assigns calls to cars is that the cars are not distributed evenly throughout the building during periods of no demand or low demand, and therefore the ability of such a system to respond to calls following periods of low demand, particularly buildings having a large number of floors, is diminished. In addition, such systems do not adequately accommodate excessive demand for service at a main landing, such as a lobby, during periods of up peak traffic, such as the start of a normal working day. Further, the ability to provide a special type of service during down peak traffic, in which the upper floors are favored to receive unassigned cars, but service is maintained throughout the building (being less severe than up peak traffic in most cases) is lacking in prior art systems which assign calls to cars.